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Dendrites (from Greek
dendron, “tree”) are the branched projections of a neuron that act to conduct the
electrical stimulation received from other neural cells to the cell
body, or soma, of
the neuron from which the dendrites project. Electrical stimulation
is transmitted onto dendrites by upstream neurons via synapses which are located at
various points throughout the dendritic arbor. Dendrites play a
critical role in integrating these synaptic inputs and in
determining the extent to which action
potentials are produced by the neuron.

Electrical properties of dendrites

The structure and
branching of a neuron's dendrites, as well as the availability and
variation in voltage-gated
ion conductances, strongly influences how it integrates the
input from other neurons, particularly those that input only
weakly. This integration is both "temporal" -- involving the
summation of stimuli that arrive in rapid succession -- as well as
"spatial" -- entailing the aggregation of excitatory and inhibitory
inputs from separate branches.

Dendrites were once believed to merely convey
stimulation passively . In this example, voltage changes measured at the
cell body result from activations of distal synapses propagating to
the soma without the aid of voltage-gated
ion channels. Passive cable
theory describes how voltage changes at a particular location
on a dendrite transmit this electrical signal through a system of
converging dendrite segments of different diameters, lengths, and
electrical properties. Based on passive cable theory one can track
how changes in a neuron’s dendritic morphology changes the membrane
voltage at the soma, and thus how variation in dendrite
architectures affects the overall output characteristics of the
neuron.

Although passive cable theory offers insights
regarding input propagation along dendrite segments, it is
important to remember that dendrite membranes are host to a
cornucopia of proteins
some of which may help amplify or attenuate synaptic input.
Sodium,
calcium, and potassium channels are all
implicated in contributing to input modulation. It is possible that
each of these ion species
has a family of channel types each with its own biophysical
characteristics relevant to synaptic input modulation. Such
characteristics include the latency
of channel opening, the electrical
conductance of the ion pore, the activation voltage, and the
activation duration. In this way, a weak input from a distal
synapse can be amplified by sodium and calcium currents en route to
the soma so that the effects of distal synapse are no less robust
than those of a proximal synapse.

One important feature of dendrites, endowed by
their active voltage gated conductances, is their ability to send
action potentials back into the dendritic arbor. Known as
backpropagating action potentials, these signals depolarize the
dendritic arbor and provide a crucial component toward synapse
modulation and long-term
potentiation. Furthermore, a train of backpropagating action
potentials artificially generated at the soma can induce a calcium
action potential at the dendritic initiation zone in certain types
of neurons. Whether or not this mechanism is of physiological
importance remains an open question.

Dendrite development

Despite the critical role that
dendrites play in the computational tendencies of neurons, very
little is known about the process by which dendrites orient
themselves in
vivo and are compelled to create the intricate branching
pattern unique to each specific neuronal class. It is likely that a
complex array of extracellular and intracellular cues
modulate dendrite development. Early candidates include: Sema3A,
Notch, CREST, and Dasm1. Sema3A may act as a dendritic
chemoattractant that aids cortical pyramidal neurons in orienting
their apical dendrites to the pial surface. Notch acts as a
neurotrophic
factor in aiding dendrite growth and branching, while CREST may
play an important role in regulating calcium dependent growth
signals. Dasm1 (Dendrite arborization and synapse maturation 1)
expression appears to be highly localized to dendrites and may have
substantial influence on dendrite (but not axon) development.